In some ways, WoW's battlegrounds are to the PvP game what raid dungeons are to PvE. Both are instanced content which you do in a group. And both offer a mix of core gameplay which doesn't vary much, together with specific rules which change from case to case. The main difference is that raids are the only way to progress in PvE at max level, but battlegrounds are supplemented by arenas. It is interesting to note, then, that WoW currently has eleven battlegrounds you can do at max level, but only one raid. Why is this?
WoW released with two raid instances and three battlegrounds. Since then, Blizzard has released over two dozen more raids. But most of those instances are no longer used, at least for their original purpose of providing endgame gear progression. By contrast, you can still play the original three battlegrounds and earn gear which is suitable for level 100 players.
It is interesting to note that this did not have to be the case. Blizzard could have used the battleground model for raids.
What if when BC came out, Blizzard had decided to simply add level 70 versions of the existing raids, dropping level 70 versions of their loot, and then place new raids after the existing ones in gear progression? Then by level 100 there would be 20+ raids to progress through.
But of course there are issues with this approach. For one thing, it would mean that power differences between fully geared raiders and fresh level 100s would become absurdly large.
And it is possible that most players would not like having to do so much old content. After all, much of what Blizzard has done with raiding in the last couple expansions has been done with the intent of removing the old progression model and helping players do current content. A model like this would run directly counter to that.
Another interesting idea would be to have level 100 versions of all the old raids, but to make them all roughly equivalent in difficulty. Then remove all drops, and have them reward "PvE Points" which could be exchanged for gear of the current tier. This brings the PvP reward model into PvE, and gives a good reason to do old content without making it absolutely mandatory.